1. Field of the Invention
This invention relates to a novel positive temperature coefficient (PTC) thermosetting polyester composition which has a low resistivity and temperature rise when used as a circuit component carrying normal current. When the current increases due to short circuit or overload, the composition temperature increases and changes to a high resistance state which limits the current to a safe value until the current is interrupted. When the current is interrupted, the material cools and reverts back to its original low resistance state. This invention has a specific application as a resettable fuse or current limiter.
2. Background Information
Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under normal operating conditions of the circuit, but are "tripped," or converted to a high resistance state when excessive current or temperature occurs. When the high resistance is caused by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in the high resistance state. The increase in resistance is accompanied by an expansion of the PTC element.
Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them are well known. For instance, PTC devices have been utilized as current limiters connected in series with the separable contacts of a current interrupter such as a circuit breaker. The PTC device prevents the short circuit current from reaching a level at which the circuit breaker becomes incapable of interrupting this current due to the inability to extinguish the arc struck between the separated contacts. Various PTC compositions and electrical devices incorporating them are shown for example in U.S. Pat. Nos. 5,049,850; 5,195,013; 4,724,417; 4,545,926; 4,685,025; 4,647,894; 4,857,880; 4,910,389; 4,774,024; and 4,775,778.
Articles have also been published in the field such as "Thermistors For Overcurrent Protection," Machine Design, Dec. 10, 1981, pp. 161-165; "Conductive Polymers," Machine Design, Oct. 22, 1992, p. 161; "Circuit Protectors," Machine Design, Dec. 12. 1991, p. 82 and "Conductive Polymers," Design News, Nov. 9, 1992, p. 99.